Improve check for logical continuation in comprehension.

[anntzer]

Excludes

sorted(obj for obj in iterator
       if some_long_cond()
          and some_other_cond())

from E127.

It is true that this also lets through some "bad" indents e.g.

(1 if a and 2 else
      b)

but at the tokenization level it is quite difficult to distinguish
between the "if" in a for b in c if d and the "if" in a if b else c,
which play quite different roles.

Closes #680.